During the manufacture of a film made of synthetic material, such as a film made of polypropylene, polyester or composed of any other thermoplastic material, the film, after its formation, passes successively through a longitudinal stretching step and a transverse stretching step.
A stretching system, configured to stretch a film made of synthetic material in the transverse direction, includes, in a known manner, a furnace allowing regulating the temperature of the film during its stretching in the transverse direction, and two stretching devices disposed on either side of the film. Each stretching device includes more particularly:                a guide rail extending at least partially in the furnace and including a first guide surface turned toward the film and a second guide surface opposite to the first guide surface,        a plurality of clamps each intended to grip the same longitudinal edge of the film, the clamps being configured to be guided in translation by the respective guide rail,        a retaining rail extending along the respective guide rail, the retaining rail including a bearing surface turned toward the respective guide rail and a retaining surface opposite to the bearing surface,        a plurality of guide elements configured to be guided in translation by the retaining rail, and        an endless chain including a succession of links hingedly mounted relative to each other and first and second articulating elements connecting in an articulated manner the links, each clamp being hingedly mounted on a respective first articulating element and each guide element being hingedly mounted on a respective second articulating element, the endless chain being configured to drive the respective clamps along the respective guide rail, and to drive the respective guide elements along the respective retaining rail, the endless chain being further deformable between a first state in which the clamps are disposed proximate to each other, and a second state in which the clamps are distant from each other.        
Advantageously, each guide element includes running rollers configured to cooperate respectively with the bearing surface and the retaining surface of the respective retaining rail, and each clamp also includes running rollers configured to cooperate respectively with the first guide surface and second guide surface of the respective guide rail. These arrangements ensure a guiding of the guide elements and of the clamps on the respective rails.
Such a stretching system is adapted to high production speeds, including when this stretching system is configured to simultaneously ensure a stretching of the film in the transverse direction and a stretching of the film in the longitudinal direction.
However, due to the considerable width of the clamps, the distance between two adjacent gripping areas of the film may turn out to be very large, particularly when each endless chain is in the second state and the clamps are distant from each other. For example, with clamps each having a width of 50 mm and a longitudinal stretching of a ratio of 8, the distance between adjacent gripping areas of the film will be 400 mm. Such a distance between clamps has the consequence that the thickness of the film is not homogeneous at the longitudinal edges of the film, which requires cutting the film at the longitudinal edges and therefore generates significant losses of products.
A solution to overcome this drawbacks consists in reducing the width of each clamp. For example, with clamps each having a width of 25 mm and a longitudinal stretching of a ratio of 8, the distance between adjacent gripping areas of the film will be only 200 mm. Thus, the use of clamps of small width allows obtaining films having a better homogeneity in thickness and this with little waste.
However, such a solution involves the use of smaller diameter running rollers in order to avoid a conflict between the running rollers of two adjacent clamps when the chain is in the first state, and therefore requires substantially reducing the displacement speed of the endless chain. This results in a substantial reduction in production speeds.